Despite offering robust mechanical properties, polymer networks suffer from a lack of recyclability, reshaping, and healability. Designing stiff and remendable polymer networks that can repair under mild conditions remains a challenge to extend their field of applications. Herein, we describe a simple approach to design a nonisocyanate-based polyurethane network featuring multiresponsiveness (to humidity and temperature) and outstanding healing properties, as obtained by combining iminoboronate and boroxine chemistry. In spite of the presence of abundant dynamic bonds, the network has a high stiffness (Young's modulus of 551 MPa) and tensile strength (11 MPa). CN iminoboronate and B−O boroxine exchange reactions at high temperature enable efficient network recycling over multiple cycles without compromising its properties. Owing to these features, 3D objects could be designed and printed. The present approach provides excellent sustainable and high-performance substitution to conventional polyurethane networks requiring the use of toxic isocyanates.
Developing intrinsic self-healing polymeric materials is of great interest nowadays to extend material lifetime and/or prevent the replacement of damaged pieces. Spontaneously humidity-sensitive healable polymer network built around dynamic covalent B-O bonds was templated by using iminoboronate-based boroxine derivatives. Taking advantage of the dynamic boroxine/boronic acid equilibrium and iminoboronate chemistry, it is possible to construct polymeric materials able to self-heal without requiring any energy-demanding external activation. Interestingly, this novel family of iminoboronate adduct-based materials can be readily produced by a relatively simple and straightforward synthesis between boronic acid and diamine-based compounds, paving the way to coatings that are self-healable at ambient humidity.
As
dynamic cross-linking networks are intrinsically weaker than
permanent covalent networks, it is a big challenge to obtain a stiff
self-healing polymer using reversible networks. Inspired by the self-healable
and mechanically adaptive nature of sea cucumber, we design a water-responsive
self-healing polymer system with reversible and permanent covalent
networks by cross-linking poly(propylene glycol) with boroxine and
epoxy. This double cross-linked structure is self-healing due to the
boroxine reversible network as well as showing a room-temperature
tensile modulus of 1059 MPa and a tensile stress of 37 MPa, on a par
with classic thermosets. The dynamic boroxine bonds provide the self-healing
response and enable up to 80% recovery in modulus and tensile strength
upon water contact. The system shows superior adhesion to metal substrates
by comparison with the commercial epoxy-based structural adhesive.
Besides, this system can change modulus from a stiff thermoset to
soft rubber (by a factor of 150) upon water stimulus, enabling potential
applications of either direct or transform printing for micro/nanofabrication.
Moreover, by incorporating conductive nanofillers, it becomes feasible
to fabricate self-healing and versatile strain/stress sensors based
on a single thermoset, with potential applications in wearable electronics
for human healthcare.
Novel melt-recyclable poly(ε-caprolactone)/cellulose nanocrystals supramolecular nanocomposite networks with shape-memory behavior have been successfully constructed by playing with UPy chemistry.
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